[Haskell-cafe] Type system

[Tillmann Rendel]

Hi Andrew,

Andrew Coppin wrote:
> Haskell has an expressive and powerful type system - which I love. It 
> also has a seemingly endless list of weird and obscure type system 
> extensions. And there are various things you can do in Haskell which 
> *require* some pretty serious type system hackery.
> 
> And yet, none of this happens in any other programming language I've 
> met. Take Java for example. In Java, a "type" is just an identifier. It 
> has no further structure than that. It does have the added semantics 
> that it refers to that class *or any subclass*, but that's about it. 
> Even taking an extreme example like Eiffel [which supports generics], 
> complex types are extremely rare.

There are a lot of Java extensions in various directions (Multimethods, 
Mixins, Virtual Classes, Aspects, Effect Typing). Some extensions have 
been included into the "official" Java Language (Anonymous Classes, 
Generics). Other extensions have lead to the creation of new languages 
different from, but related to Java (Scala). A lot of research teams, 
both academic and industrial, are working on new extensions of the Java 
Language. Such extensions may come as libraries, preprocessors or 
compilers, they may reuse, extend or substitute the run time system.

 From my limited view, the Java research community seems to be bigger 
then the Haskell research community. But I may be wrong here, and the 
meaning of a "big community" isn't clear at all, so that's not the point.

Clearly, Haskell feels like a research language, and Haskell hacking is 
like pushing the border into unexplored territory, while Java feels like 
  a lot of boilerplate without actual content, and Java development is 
like taking care of paperwork.

Why is this the case? I see four reasons all working together:

First, Java is designed for the software industry, while Haskell "avoids 
success at all cost". This means that we have 90% research people 
working with Haskell, and 0.01% research people working with Java, *even 
if there are more Java researchers as Haskell researchers*. It's the 
absence of non-researchers which makes Haskell a research language, not 
the abundance of researchers.

Second, Java extensions tend to be incompatible with each other, since 
they typically consist of an modified compiler, while Haskell extensions 
tend to get integrated into GHC or lost forever. With GHC as de-facto 
standard compiler, we have a number of extensions available for every 
productive Haskell system.

Third, the theoretical base of pure functional languages is relatively 
clear and easy to understand, while the theoretical base of imperative 
object-oriented languages is still widely unexplored and hard to grasp. 
This means that formal, scientific papers about Haskell are more likely 
to be user-understandable then formal, scientific papers about Java. 
Ever seen one of the formal calculi proposed as semantic core of the 
Java language?

Fourth, Haskell is just better. (Had to include this one to avoid being 
victim to a flame war). But hey, it is my personal opinion too.

   Tillmann